The optotypes fields allow you to select both the letters to be displayed, and the order of letters on each line. You cannot change the array nor order of the lines displayed here. Custom Optotypes and Alphabets can be installed.
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Optotype: Select from the standard optotypes and those you have installed yourself.
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Alphabet: The available options are dependent on the OPTOTYPE selected above. SnellenU and SloanU use a wider selection of letters, but letters may not be of equal readability. BS4724.3 and ETDRS are standard alphabets using fewer letters, but with equal readability. Letters can be shuffled to prevent learning - see below